100 Most asked questions by Chemical Engineers

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Wednesday, February 6, 2013

Seal Drum Problems for Vacuum Systems

            Likely of the problems I encounter in refinery vacuum systems are due to difficulties with the seal drum. For example:

  1. The seal legs are too short. This causes the bottom portion of the seal leg to be exposed to the vapor, which is then drawn up into the condenser, which no longer drains.
  2. The seal legs are too long. Many seal drums have the bottom of the seal leg 4” above the bottom of the vessel. Between turnarounds, sludge covers the bottom of the seal leg and the condensers stop draining.
  3. Poor level control in seal drum. The seal baffle should prevent loss of the liquid seal. However, a high level will cause condensate back-up.
  4. Corrosion of seal leg inside drum. The corrosion is biological in nature. The holes in the legs above the overflow baffle will then cause the leg to lose its seal and condensate drainage will stop.

Sunday, January 27, 2013

Troubleshooting Process Problems

             My mother could never understand my work.

            “Mom,” I explained for the hundredth time, “I’m a field troubleshooter.”

            “So. Norman, how much do they pay you? It’s a good living?”

            “I do okay.”

            “Mrs. Silverman’s son, Bernie. He’s a big lawyer in New York. He charges by the hour. One hour is $200!”

            “Yeah mom. I do okay. I charge $2,400 a day.”

            “And who would pay so much? Only crazy people! If only you had become a doctor – like your cousin Samuel. He has a big practice in Miami.”

            “Mostly I work for Exxon, Chevron, BP, and Mobil. They have lots of problems and lots of money. As long as I can solve their problems, I can charge anything. The people who hire me aren’t spending their own money. They don’t care.”

            And then for the hundredth time, the same inevitable question.

            “Norman. Don’t these companies have their own engineers? They should hire engineers themselves. Mrs. Howotiz’s son, Nathan. He’s an engineer like you. Maybe you could tell Exxon. They could hire him. He’s been looking for a job since summer. He’s such a nice boy.”

            “Look mom. I’ve explained all this to you before. Exxon and Chevron have thousands of engineers already. But they’re office engineers; they’re telephone engineers; they’re computer engineers. They spend their days attending meetings and talking on their cell phones and sending emails. They’re professionals.”

            “So, professional is good. Mrs. Goldberg next door always says, ‘If only my daughter Sarah could marry a professional like your son Norman, I would die happy.’”

            “Mom. I’ve told you before. I’m not a professional. I’m a worker with a trade. My trade is applying Chemical Engineering principles in the field to solve refinery process problems.

            “So Mr. Worker, you want some lunch? Look at you. You’re all skin and bones. I’ll heat a nice bowl of chicken soup for you.”

            “You see mom. Those engineers who work for the big oil companies – they’re all pretty smart. They’re good engineers.”

            “But not as smart as my son. Even Mrs. Silverman in 4-C says your son Norman is really smart, but too skinny. You like noodles in your soup?”

            “It’s not a matter of being smart. Or having an engineering degree, or having lots of experience. Those things don’t help solve problems all that much. It’s something else.”

            But mom was no longer listening, “Where’s your father? He must have fallen asleep in the park again.”

            “You see mom. It’s a matter of determination. When I have a refinery process problem to solve, it’s a matter of life and death. Just last week, I risked my life climbing a rickety scaffold to get a skin temperature on a jet fuel draw-off line. If I need a sample, and it’s against the plant’s safety practices to get such a sample, I’ll wait until nobody is watching, and get the sample myself. If I’m working on a problem, and I’m tired, hungry, and cold, I still keep going. I’ll never give up. Better death than defeat. The Shell and Conoco engineers don’t look at their jobs my way. They’re bound by safety practices and company rules as to what engineers and operators are allowed to do. But rules never apply to me. I can do anything.”

            “But why is that?” mom asked.

            “Because I’m determined.”

            “Norman. Go and look for your father. He’s probably playing cards with the Mexicans in the park again. Did you know that your father speaks Spanish?”

            “Yeah mom. I know. I’ll go and look for him. Save the soup. Don’t give it to the kids.”

Distillation-Effect of Feed Preheat

           Increasing feed preheat to a distillation tower typically makes fractionation worse, assuming that the reflux rate is fixed. That means, that if the concentration of the heavy component in the overhead product is held constant, then the concentration of the light product in the tower’s bottom product will increase.

            But why?

            The reason is the reboiler must decrease. If the reflux rate is held constant, and the concentration of the heavy component in the overhead is held constant, then the condenser duty (i.e., heat removal) is also constant. Thus, if I increase the heat input with the feed, it follows that the reboiler duty must go down to maintain the tower heat balance.

            A decrease in the reboiler duty will also decrease the vapor flow through the trays between the feed tray and the bottom tray. As this vapor flow goes down, the stripping section trays don’t work as hard. Or, we can say that the stripping factor is reduced. Or, perhaps, the rate of tray deck leakage increases. Either way, the amount of the lighter component slipping into the bottom of the distillation tower will increase.

            What then are the benefits of increased feed preheat? There must be such benefits, otherwise most of our towers would not have feed preheaters:


  • Saves Energy – The feed preheater normally uses waste heat or low pressure steam, with little value. This saves more valuable reboiler steam.
  • Supplements Reboiler Capacity – If the tower is not condenser limited, but limited by the capacity of the reboiler, then more feed preheat will permit higher reflux rates, and hence better fractionation efficiency.
  • Stops Flooding in Bottom Section Trays – If the tower is limited by flooding or entrainment in the bottom stripping trays, then more feed preheat, which reduces the reboiler duty, will improve fractionation. This happens because the vapor velocities will go down, as the feed preheat goes up.


So, depending on the hydraulic capacity of the stripping trays, feed preheat can make fractionation better or worse.

Effect of Steam Superheat

            You would think superheating steam would always increase the usefulness of the steam. This is not always true. A few examples:

·         Steam superheat is bad for amine reboilers. The superheated steam increases the shell side tube temperature and accelerates H2S corrosion.

·         Steam superheat is bad for jets. The superheated steam increases the volumetric flow through the steam nozzle, and thus reduces the height flow of steam.

·         Steam superheat is neutral for reboilers in general. The LMTD is not increased by using superheated steam. It’s the saturation temperature which determines the LMTD.

·         Steam superheat is slightly helpful when stripping a product. But the effect is so minor that I typically ignore superheat in my steam stripper calculations.


The real value of superheating steam is the guarantee that it is dry. Dry steam is critical for steam strippers, jets, and turbines.